Gregor Mendel & Punnett Squares

11.1-11.2 Gregor Mendel&Punnett Squares

Pages 262 - 266

PD -old

Goal 1

Describe how Mendel Studied inheritance in peas

Austrian Monk 1800’s

Taught High School

Managed monastery gardens

Pea plant observations

1. True-breeding plantsa) Self pollination =

offspring identical to parents

2. Cross pollination used to study inheritancea) Tall plant x short plant

b) Green seeds x yellow seeds

Goal 2

Summarize Mendel’s conclusions about inheritance

• Results of Mendel’s experiment– Cross pollination of true breeding parents (P)– Offspring (F1) only show one trait– No “blending” of traits

Tall plant x short plant = all tall plants

P P F1

• Mendel’s 1st conclusions about inheritance

1. Inheritance determined by factors passed from one generation to the next

2. Each trait determined by one gene that occurs in two different forms (alleles)

3. Trait of height located on one gene as either short or tall alleles

Goal 3

Explain the principle of dominance

• He tried again with F1 generation– Cross pollination of F1 offspring– ¾ F2 offspring show one trait ¼ F2 show

“hidden” trait

F1 x F1 = F2

Tall plant x Tall plant = 3/4 tall plants

¼ short plants

Creative commons Image credit: Madprime

Principle of dominance states:

some alleles are dominant and others are recessive

– If dominant allele present, it will be expressed

– Recessive alleles only expressed if NO dominant allele present

Goal 4

Describe what happens to alleles during segregation in

meiosis

• Alleles separate during meiosis

• Alleles segregate into one allele per gamete

• Two types of gametes can be produced

• Alleles pair up randomly when gametes unite during fertilization

Example on the board…

Goal 5

Use Punnett Squares to:

determine genotypic and phenotypic ratios

of a crossto

Predict outcomes

• Phenotype = the expressed physical trait– What can be seen (brown hair)– Dependent on genotype

• Genotype = the alleles of a genetic code– Genetic code (one maternal, one paternal)

• Homozygous = alleles are the same (BB or bb)• Heterozygous = alleles are different (Bb)

• Recessive phenotype– Genotype MUST be homozygous recessive– Genotype bb expresses phenotype blond hair

• Dominant phenotype– Genotype may be homozygous dominant

• BB expresses brown hair

– Genotype may be heterozygous• Bb also expresses brown hair

• Genotypes are used to create Punnett Squares and compare possible combinations of gametes

Parent 1 gamete possibilities

Parent 2 gamete possibilities

Possible

Combination

Possible

Combination

Possible

Combination

Possible

Combination

B

b

b

B

Parent 1: homozygous dominantParent 2: homozygous recessive

BbBb

Bb Bb

B

b

B

b

Parent 1: heterozygousParent 2: heterozygous

bbBb

BB Bb

B

b

B

b

Genotypic ratio of offspring 1BB : 2Bb : 1bb

Phenotypic ratio of offspring 3 brown hair : 1 blond hair

(75% chance) (25% chance)

bbBlond hair

BbBrown hair

BBBrown hair

BbBrown hair

?

?

?

?

Parent 1: heterozygous (Bb)Parent 2: homozygous recessive (??)

????

?? ??

B

b

b

b

Parent 1: heterozygous (Bb)Parent 2: homozygous recessive (bb)

bbBb

Bb bb

B

b

b

b

bbBb

Bb bb

Genotypic ratio of offspring ?? : ??

Phenotypic ratio of offspring ?? : ??

B

b

b

b

bbBb

Bb bb

Genotypic ratio of offspring 2Bb : 2bb

Phenotypic ratio of offspring 2 brown hair : 2 blond hair

(50% chance) (50% chance)

Thanks to the following people who have generously shared images on Flickr

creative commons:

Unertlkm… pea plant

Z e n… butterfly and pea bloom